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Automating radiator based heating system

Our house has a radiator heating system, fired by a large Marshall wood burner.

I’m looking to improve this heating system to achieve:

some level of heat storage – so that we don’t have to run the wood fire whenever we want heat

a way for that storage to stay reasonably warm for 4-5 days, so we can go away and have the heating automatically run before we return home

thermostat control on the individual radiators so we can hold a temperature, rather than adjusting temperature through how vigorously we stoke the fire

zoning of the rooms, so that those rooms not in use can not consume heat

allow me to run the boiler hard, rather than damping it down, which reduces pollution and increases heat output from a given weight of wood

the boiler also runs the domestic hot water (DHW) in winter, currently that’s an open cylinder rather than mains pressure, it’s a bit small, and takes forever to get hot water to the kitchen

the thermostat on the boiler that controls the pump is slow to cycle, leading to the boiler overheating and boiling the water, rather than heating it. This is wasteful of heat.

Current planning for the solution comes in multiple parts.

Convert the boiler to drive a mains pressure hot water, and a buffer tank. This means we can run the boiler hard, with all the heat going directly into the DHW and the buffer tank, irrespective of current heat demand. This would be combined with a new controller unit for the pump, with an electronic thermostat so that it cycles much faster

Implement a new pump circuit (reusing one of the old solar hot water pumps) to drive the radiators from the buffer tank

Convert the DHW system to mains pressure, allowing a larger cylinder (the current one is elevated a bit, a new one could go all the way to the floor). This would be combined with a ring main + supply pump, and lagging on the hot water pipes, so that there is always hot water near the kitchen

Implement a control system that can measure the temperature in each room, adjust the radiator valves, control the circulation pump run-time, and implement zoning and set points for daytime and nighttime use

For a buffer tank, I’m looking at a Fiorini buffer tank, similar to this 1000L one, but preferably in 2000L. The main site suggests they’re available. This tank has no coils in it, so reasonably low tech. It’s very large, review on the web suggests that a 2000L tank with a usable 40 degree temperature delta (40 degrees to 80 degrees) would have 100kwh of heat storage. In a steady state that’d probably heat our large and somewhat draughty house for about 1 day.

This would give a boiler loop that looks something like:

We’d need a new pump controller, I haven’t researched those yet.

The radiators are then driven from the buffer tank. If the buffer tank is mostly cold that’s still OK, as the hot water goes in the top from the boiler, and then comes out immediately to go to the radiators.

For a tank, I’d be looking at something with two electric elements plus a heating coil, perhaps something like this Rheem 300L stainless jobby.

The aim is that we have a coil that heats from the boiler (but doesn’t mix the heating water with the DHW), then two electric elements – one driven from night rate and one from normal power. The night rate one would be set to 80 degrees, the day rate one to about 50 degrees. That way we mostly use night rate or boiler energy, but can use day power when we have lots of guests or otherwise run out of hot water.

We’d combine this with a ring main core hot water loop, driven by an on-demand pump such as this grundfos one, which information here suggests uses 5-8w when running, and auto adapts to when you use water such that it costs only a couple dollars a year to run.

This means picking the main loop of hot water, and installing a return line to the DHW cylinder, then lagging that entire loop so that it doesn’t lose much temperature. Some of our piping is plastic, I’d rather it were all copper, so we’d replace that the same time.

Finally, since this hot water will run at up to 80 or 90 degrees when the heating is running hot, it needs tempering valves on every tap so that the actual available hot water temperature is about 60 degrees in the kitchen(s), and 50 degrees everywhere else. As noted in the link this allows storing a smaller amount of very hot water, and mixing with cold at point of use to reduce your overall hot water usage (and improve safety). These valves will also allow constant water output temperature from the ring main, even as temperature fluctuates due to cooling in the pipes.

Control System

For the control system I’m looking at OpenHab, combined with the Max wireless control system.

If I use the Max CUL binding, I can get directly at the valve controls. I’ll write more on this, but in theory what I’d want is:

The most basic Max valves, as I won’t be using the programming capability. These provide a thermostat (they know the temperature of the room near themselves), and allow setting of a desired temperature plus report back the valve open position each time it changes

Temperature sensors on the buffer tank – top, middle and bottom. This allows me to turn off the pump when we run out of heat, and to estimate how much heat is stored in the tank. Need to use the openHab one-wire binding

Temperature sensors on the return from the radiators, telling me if the return water is hot (indicating we should stop circulating)

Logic that adjusts the set point on each for night time, for zoning rooms on and off

Logic that assesses how long to run the pump for based on when it last ran, how far away we are from the set point, how cold it is outside, and how warm the return water is. The aim is to let the bottom of the radiators get quite cold to maximise heat extraction, but maybe that’s not a good aim – maybe we should aim to keep the radiators hot to maximise the warming, we can always reuse the water later so long as it goes back to the buffer tank

Some of the radiators need to not have controllers, in case we accidentally run the pump when all the radiators are off. Probably the towel rail in the main bathroom would be the one that had no thermostat (maybe in the other bathroom too – who needs a thermostat on a towel rail anyway….just means you can’t zone it off)